A disk drive is a digital data storage device that stores information within concentric tracks on a storage disk. The disk is coated on both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of the disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetically polarized transitions into the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer.
The write and read signals are analog signals that define what is referred to as a data band. The data band is a range of frequencies in which the write and read signals are contained.
FIG. 1 illustrates a conventional disk drive 10. The disk drive 10 includes a disk 12 that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 mounted to a flexure arm 22, which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. A voice coil motor (VCM) 28 is coupled with the actuator arm assembly 18 to radially position the transducer 20 relative to the disk 12. The spindle motor 14, the transducer 20 and the VCM 28 are coupled to drive electronics 30 mounted to a printed circuit board (not shown). The drive electronics 30 typically include a preamplifier, a read channel, a servo control unit, a microprocessor-based controller, and a random access memory (RAM).
The disk drive 10 includes at least one and typically multiple disks 12, each with one or two recording surfaces. An actuator arm assembly 18 is provided for each recording surface of each disk 12.
The transducer 20 is a dual element transducer that includes separate read and write elements. Single element transducers usually contain a single inductive element that performs both read and write functions, whereas dual element transducers usually contain a magneto-resistive (MR) read element and an inductive write element. The MR read element can be a conventional magneto-resistive element, a giant magneto-resistive (GMR) element, or a similar component.
Since the transducer 20 is a dual element transducer, the read and write elements can be optimized for their respective functions. For example, MR read elements are more sensitive than inductive read elements to small variable magnetic fields, which permits MR read elements to read much fainter signals from the disk 12. Employing an MR read element permits data to be more densely packed on the disk 12.
MR read elements generally include a strip of magneto-resistive material between two magnetic shields. When properly biased, the resistance of the magneto-resistive material varies almost linearly with an applied magnetic field. During a read operation, the MR strip is positioned above (or below) a desired track within the varying magnetic field caused by magnetic transitions on the track and a constant bias current is passed through the strip. By Ohm's law (V=IR), the variable resistance and the constant bias current of the MR strip result in a variable voltage across the MR strip that is proportional to the variable resistance. That is, V+δV=I(R+δR). Therefore, the variable voltage is representative of the data stored within the desired track. The variable voltage provides an analog read signal which is then amplified by the preamplifier, processed and converted into digital form by the read channel, and transferred by the controller to a host computer.
FIG. 2 is a diagrammatic representation of an air bearing surface of the transducer 20 which faces the disk 12. As is seen, the transducer 20 includes an inductive write element 34, a write gap 36, a first shield 38, a second shield 40, a read gap 42, and an MR read element 44.
During a read operation, the magnetically polarized transitions previously written onto the disk 12 are read by the MR read element 44. The first and second shields 38 and 40 form the read gap 42 which serves to focus the flux from the magnetically polarized transitions onto the MR read element 44 by shielding the MR element 44 from other sources of magnetic flux (e.g., sources of magnetic flux not associated with the particular location from which information is being read). In other words, the first and second shields 38 and 40 shunt extraneous magnetic flux away from the MR read element 44 as reading occurs.
During a write operation, variable current is applied to write coils (not shown) in the transducer 20 which induce magnetic flux across the write gap 36 between the write element 34 and the first shield 38. The write element 34 and first shield 38 act as poles for an electromagnet which induces the magnetic flux across the write gap 36 that records magnetically polarized transitions on the disk 12. Furthermore, since the magnetic flux in the write gap 36 has relatively high intensity, and the MR read element 44 is in close proximity to the write gap 36, a large amount of the magnetic flux across the write gap 36 affects the MR read element 44 during a write operation. Consequently, the MR read element 44 is typically not used to read data from the disk 12 during a write operation.
FIG. 3 is a simplified diagrammatic representation of a cross-sectional view of an air bearing slider 46 that includes the transducer 20 flying above a disk surface 48 of the disk 12. The slider 46 is located at the distal end (opposite VCM 28) of the actuator arm assembly 18. The slider 46 includes a leading edge 50 and a trailing edge 52. The transducer 20 is located proximate to the trailing edge 52. The distance between the transducer 20 and the disk surface 48 is known as the flying height (hf) of the transducer 20.
The transducer 20 and the slider 46 form a head 60. A head/disk interface 62 is defined by the head 60 and the disk 12. More specifically, the head/disk interface 62 comprises the disk 12, the slider 46, and, during normal read and write operations, a flying height gap 64 between the head 60 and the disk surface 48 of the disk 12. The flying height gap 64 is a three-dimensional space defined by the shortest distance between each point on the head 60 exposed to the disk surface 48 and a corresponding point on the disk surface 48 along a line perpendicular to the disk surface 48. The volume of the flying height gap 64 typically varies as the distance between the head 60 and the disk surface 48 changes during disk drive operations.
During operation of the disk drive 10, a preload force is applied to the head 60. The preload force is the composite forces applied by a number of sources. In particular, the flexure arm 22 applies a mechanical spring force to bias the head 60 towards the disk 12. At the same time, the disk 12 is rotated in the direction of arrow A from the leading edge 50 to the trailing edge 52. The slider 46 is aerodynamically designed so that, when the disk 12 rotates at its normal operating speed, a small cushion of air between the slider 46 and the disk surface 48 forces the slider 46 (and hence the head 60) away from the disk surface 48 against the spring force applied by the flexure arm 22. The passive mechanical and aerodynamic forces that hold the head 60 away from the disk 12 are referred to as an air bearing system. Ideally, the air bearing system is designed to maintain the head 60 at a nominal flying height. However, several factors can affect the actual flying height of the head 60 of a given disk drive 10.
Initially, the flying heights of two disk drives 10 that are theoretically identical are often different upon manufacture and/or during use. The tolerances of the mechanical parts used during the assembly of different disk drives 10 can differ. Different tolerances can lead to different preload forces for different individual disk drives 10. In addition, different operating environments can result in different flying heights. For example, the flying height will typically be lower at high altitude than at sea level.
The slider 46 is shaped so that fly height is less susceptible to variations in preload forces and operating environments. However, shaping the slider 46 based on fly height considerations can result in a slider 46 that is not optimized for other considerations such as reducing debris collection. In addition, different sliders 46 may be required for operating parameters, such as disk revolution speed, associated with different disk drive products. The use of different sliders for different products typically results in increased manufacturing costs.
In addition, even for a particular disk drive 10 under normal operating environments, the flying height can change during disk drive operation for various reasons. First, the slider 46 may strike contaminants 54 on the disk surface 48 which temporarily stick to the slider 46 and change its aerodynamic characteristics. Second, the slider 46 may strike and bounce off contaminants 54 or perturbations 56 in the disk surface 48. In addition, gradual accumulation of debris onto the slider 46 can increase the flying height.
FIG. 4 is a simplified diagrammatic representation of a cross-sectional view of the slider 46 during high fly writing. As shown in FIG. 4, the flying height (hf) of the transducer 20 exceeds a predetermined nominal flying height (hnom) by a distance x. In other words, hf=hnom+x. The flying height (hf) of the transducer 20 is related to the flying height gap 64.
The performance of the disk drive 10 will depend, to a large extent, on whether the flying height of the head 60, and thus the transducer 20, stays within a predetermined flying height range. For instance, if the flying height of the transducer 20 is too low then the head 60 might engage in excessive contact with the disk surface 48. This contact may damage the transducer 20 and/or the disk 12 or cause excessive debris or lubricant from the disk surface 48 to accumulate on the head 60.
On the other hand, if the flying height of head 60 is too high, then data errors might occur during read and write operations. More particularly, if the transducer 20 flies too high during a read operation then the transducer 20 might not adequately sense the magnetic polarity transitions on the disk 12, and if the transducer 20 flies too high during a write operation then the transducer 20 might not adequately induce the magnetic polarity transitions onto the disk 12.
When the write element 34 is higher than the predetermined maximum flying height, the magnetically polarized transitions (data) written onto the disk 12 are faintly or poorly written. Consequently, the poorly written data is not properly read by the MR read element 44 when such data is sought to be recovered. In addition, since the write element 34 is higher than the predetermined maximum flying height, the write element 34 may also write over parts of tracks adjacent to the track onto which the data is sought to be written. This may render previously written data on the adjacent tracks to be unreadable.
Unexpected changes in flying height can thus result in performance and/or reliability degradation of the disk drive 10. One purpose of the present invention is to alleviate the problem of high fly writing which occurs when the disk drive 10 performs a write operation while the transducer 20 flies too high.
Another potential problem with conventional head/disk interfaces is the electrostatic charging of the head/disk interface. Several known phenomena are capable of causing an electrostatic charge to build up on the disk 12 and/or head 60 during disk drive operations. For example, if the head 60 comes into contact with the disk 12, an electrostatic charge can be induced in the disk 12 and/or the head 60. Repeated contact between the head 60 and the disk 12 can cause the electrostatic charge to accumulate over time. Electrostatic charge on the head 60 and the disk 12 can cause a difference in electrical potential between the head 60 and the disk 12. Such potential differences, especially as the electrostatic charge accumulates, can result in current flow between the head 60 and the disk 12 if the head 60 contacts the disk 12. Current flow between the head 60 and the disk 12 can result in a change in the electrochemical characteristics of one or both of the head 60 and the disk 12 at the head/disk interface 62. Such electrochemical changes can possibly lead to a disruption of disk drive operations, especially with higher density disks.
In addition, conventional air bearing systems tend to resonate at a natural oscillating frequency, especially when the head 60 is flying very close to the disk 12. For example, in some disk drives under certain conditions, the head will move towards and away from the disk (i.e., in the z-direction) at a frequency of approximately 200 KHz. Such head oscillations can lead to unpredictable behavior of the disk drive.
Accordingly, a need exists for a disk drive with a dual element transducer that reduces the likelihood of a potential difference at the head/disk interface, monitors flying height during a write operation so that appropriate measures can be taken when high fly writing occurs, and/or controls flying height to reduce high fly writing and/or head oscillations.